It is well known that during the course of normal operations, aircraft can encounter atmospheric conditions which cause an accretion of ice to develop on various aircraft surfaces or structures, particularly along leading edges in the airstream. In addition to detrimentally increasing aircraft weight, this ice can actually cause damage to some aircraft structures, such as propulsion engines, ram air turbines (RATs) and wingtip vortex turbines. For example, if ice builds up on the nose cone of a RAT and then breaks off in mass or in large chunks, the RAT blades can be bent or broken and/or an out of balance condition created in the RAT.
Previous aircraft have been equipped with anti-icing and/or de-icing equipment capable of operation during flight. Various different methods have been employed by this equipment, including ducting hot liquids or engine bleed air to the affected surfaces, embedding electrical resistance heaters in the affected surfaces, and using mechanical shields or inflatable boots adjacent the affected surfaces.
While this prior equipment has performed satisfactorily for some of the surfaces affected by icing, in other instances it has not. For example, applying such equipment to nose cones of rotating and non-rotating aircraft structures has been found to be unsatisfactory and/or impractical due to difficulties encountered in providing ducting, electrical connections, or mechanical linkages between the nose cone and the source of heat, air or power needed to prevent or remove ice. To better understand these difficulties, specific illustration is provided with respect to a typical RAT.
Aircraft RATs are often employed as emergency power generators, usually in stowage until use is required. As an emergency system, reliability and durability in the emergency environment is of critical importance. RATs need to be quickly operable in a wide range of altitudes and in various weather conditions. Thus, in low altitude or low airspeed deployment, as for example, immediately after take off, start up drag torque on RAT blade rotation should be minimized so as to avoid requiring larger blades for fast start up. Moreover, since little or no additional or supplemental power may be available to the RAT during aircraft emergencies, RATs should ideally be self-sustaining or very efficient in utilization of what supplemental power is needed. Further, since aircraft emergencies are relatively rare occurrences and RATs spend virtually their entire useful life as "excess baggage", it is often important that RATs be lightweight, compact in size and inexpensive in order to reduce aircraft cost and operating expenses.
In seeking to prevent ice build up on a RAT when it is deployed out of stowage, it becomes readily apparent that many of the anti-icing/de-icing apparatus used with respect to other aircraft surfaces would substantially increase the weight and cost of the RAT as well as detrimentally affect RAT reliability and power generation performance. For example, one type of arrangement that has been suggested for use in related structures includes electrical resistance heating elements. U.S. Pat. No. 2,488,392 shows the use of such devices in propellers of the aircraft main power plant. In that case, a dynamoelectric device is placed within the propeller hub to provide electric current to blade pitch control motors and to the heating elements located in the leading edges of the propeller blades and the spinner. The electric current is transferred to the heating elements, which are subject to rotation with respect to the dynamoelectric device, through the use of slip rings. However, slip rings have generally been found to be undesirable for use in equipment, such as RATs, which require high reliability and long term maintenance free operation due to the inherent wear of the slip rings incident with their operation and higher start up drag torque.
Further, RATs generally employ a less complex mechanical actuation and governing device to adjust blade pitch. Including the dynamoelectric device of the '392 patent would, thus, significantly increase the weight and cost of a RAT, even if slip ring reliability and start up drag torque could be simultaneously improved. Using existing aircraft power sources instead of that dynamoelectric device to provide current to the heating elements is likewise undesirable because of power conversion losses and the limited or non-existent availability of that supplemental power during emergencies.
Other rotating and non-rotating aircraft structures having nose cones face some of the same anti-icing/de-icing difficulties as illustrated above with respect to RATs. Furthermore, as a general matter, such difficulties can similarly be faced by a variety of devices exposed to a fluid stream in which temperature affects operating performance. Accordingly, it is an object of this invention to provide an improved arrangement for applying heat to external aircraft structures. Other objects include providing:
1. an arrangement for preventing ice accretion on aircraft RATs over an extended period of operation in an emergency environment,
2. a lightweight and inexpensive device for heating structures exposed to a fluid stream,
3. a reliable anti-icing/de-icing structure which can be readily incorporated into existing RAT designs without significantly increasing RAT weight or size,
4. a self-contained apparatus for heating external aircraft components which does not require a supplemental power supply,
5. a new and improved RAT, and
6. a self-contained arrangement for generating heat to be applied to external surfaces, such as nose cones, exposed to an airstream which is reliable under emergency operating conditions of aircraft and maintenance free during long periods of stowage between emergency uses.
These and other objects of the present invention are attained by apparatus for providing heat by electromagnetic induction between relatively rotating components. In RAT applications, part of the nose cone is formed from electrically conductive material which is in thermally conductive relation to the nose cone exterior. A magnetic field is created by components that are spaced apart from the electrically conductive material and relatively rotatable with respect to that material. The electrically conductive material intersects the magnetic field, at least in part, such that electrical currents, such as eddy currents, are created in the electrically conductive material by relative rotation with respect to the magnetic field. These eddy currents create heat which is conducted to the exterior surface of the RAT nose cone. Preferably, the magnetic field is created by pairs of spaced-apart permanent magnets mounted on a support plate inside the RAT hub and immediately behind the nose cone. The effect of the magnetic field in heating the nose cone can be increased by use of magnetically permeable material to form the support plate and/or part of the nose cone adjacent or integral with the electrically conductive material.
Other objects, advantages and novel features of the present invention will now become readily apparent upon consideration of the following drawings and detailed description of preferred embodiments.